Wide band frequency modulator, of the solid state type, with linear charac-teristics



3,371,289 ATE Feb. 27, 1968 A. LUNA ETAL NCY MODULATOR. OF THE SOLID STTYPE, WITH LINEAR CHARACTERISTICS WIDE BAND FREQUE Filed July 30, 1965 2Sheets-Sheet 1 B n04 m :Q L .11 (Q 21 m m 9 8 7 R R w R W R TA- 2 4 I ll I I l I l T 3 T I T Pub 5. 4 I 3 C B l T I 5 6 HQ" 0 R e R r T h R 4 h4 11 1 mm c R I l 3 Q MR a b I I F R Q G s l O U I 2 E l. I E R R x h M9 2 7 Am. R u R c huh B m 4 l 5 3 R T l R T 3 m J I I a q 3 2 I R I RFIG. 3

FIG. PRIOR ART FIG. 2

INVENTORS AGOSTINO LUNA ROBERT) CAF SI {*7 ATTY.

Feb. 27, 1968 A. LUNA ETAL 3,

WIDE BAND FREQUENCY MODULATOR, OF THE SOLID STATE TYPE, WITH LINEARCHARACTERISTICS Filed July so, 1965 2 Sheets-Sheet 2 FIG. 6

INVENTORS AGOSTINO LUNA BERTO FISSI United States Patent Ofilice 2Claims. (Cl. 332-14) ABSTRACT OF THE DESCLOSURE A. frequency modulatoremploys a free-running, astable multivibrator as a variable frequencyoscillator. Linear charging and discharging of the emitter couplingcapacitor of the multivibrator is insured by a pair of constant currentgenerators connected thereacross, the output of one of the generatorsbeing variable in response to the modulating signal. A variable loadcircuit connected to the collector of one transistor of themultivibrator and to the base of the other transistor includes a diodewhich is conductive with the said one transistor to define one triggerlevel and a reactive circuit connected to the diode and said onetransistor provides a second trigger level across the diode when thediode becomes non-conductive.

This application is a continuation-in-part of our United States patentapplication, Wide Band Frequency Modulator of the Solid State Type withLinear Characteristics, Ser. No. 407,964, filed Nov. 2, 1964.

In the parent application we decribe a frequency modulator whosemodulation characteristic is made substantially linear by substitutingfor the frequency fixing element of its multivibrator a special circuit,which in addition to fixing the frequency of the multivibrator, variesits own characteristics so as to make its modulation characteristiclinear.

According to one embodment of the invention described in the parentapplication a multivibrator comprising two transistors has substitutedfor the load resistance of one of these transistors a linearizingcircuit which includes a first resistance inserted in series with adiode, the combination of which is connected between the collector ofthe transistor and a terminal having a predetermined biasing potential,and a second resistance connected between the junction of the firstresistance and the diode and another terminal having anotherpredetermined biasing potential.

It has been found that a linearizing circuit as mentioned above has thedisadvantage that its linearization is subject to the diodecharacteristic linked to the reverse recovery time. In such a circuit itis necessary to employ a diode havig a high reverse recovery time whichperforms the dual functions of defining in the conductive stages thefirst of two voltage levels between which the multivibrator oscillates(minus the voltage drop at the terminals of the resistance in seriestherewith) and in the blocked stage by means of the behavior time of thereverse current, a phenomenon linked to the reverse recovery time,generating the potential of the junction between the diode and the firstresistance through the second linearizing resistance. The second levelis therefore characterized by a behavior with frequency favorable to theconditions of linearity.

It has been further found that it is possible to free the linearizationof the circuit from the characteristics of the diode, which permitsfirst of all, the employment of fast acting diodes of a greater varietyof types. In addition 3,37LZ39 Patented Feb. 27, 1968 there is obtaineda substantial improvement with respect to the noise introduced by themodulator.

According to the above findings, the diode retains the indispensablefunction of defining, in its conductive phase, and always less thevoltage drop at the terminals of the first resistance which isequivalent to the above noted first resistance, the first of twotriggering levels of the multivbrator. By means of a reactive circuitwhich forms a linearizing circuit, a second triggering level is obtainedby creating a transient voltage at the terminals of the completelyblocked diode.

It is the primary object of the invention to provide an improved solidstate wide band frequency modulator arrangement.

A more particular object of the invention is to provide a wide bandfiequency modulator of the solid state type having improved linearcharacteristics.

The invention is realized in one particular embodiment thereof ascomprising a multivibrator formed of two transistors, one of which has alinearization circuit including essentially a first resistance in serieswith the diode inserted between the collector of one of the transistorsand a terminal having a predetermined biasing potential, and a secondresistance inserted between the junction of the first resistance and thediode and a second terminal having another predetermined biasingpotential, as we described in our above-mentioned parent application andimproved by employing a second resistance connected to a source ofsupply through a reactive circuit which comprises a third resistance andan inductance connected in parallel, and further a capacitanceinterposed between the point of connection of the second resistance withthe reactor circuit and ground.

The invention will be best understood from the following description,given purely by way of non-limitive examples of two forms of itsexecution, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic representation of a prior art transistormultivibrator;

FIG. 2 is a graphical representation of the behavior of frequency as afunction of the current of one of the multivibrator transistors of FIG.1;

FIG. 3. is a schematic representation of the multivibrator of FIG. 1equipped with a linearizing circuit according to the teachings in theparent application;

FIG. 4 is a schematic representation of a two transistor multivibratorwhich employs a linearization circuit according to our above-mentionedpatent application, wherein, for simplicity, the constant currentgenerators are symbolically shown and the voltage dividers have beenomitted;

FIG. 5 is a schematic representation of an embodiment of the inventionequipped with another linearizing circuit; and

FIG. 6 is a schematic representation of another embodiment of theinvention with yet another linearizing circuit.

The multivibrator of FIG. 1 and its operation are well known in the artand will not be discussed herein other than to note that the frequencyof the alternate blocking and conductive phases of transistors T and Tis basically determined by capacitance C and resistance R Transistors Tand T serve as constant current generators with current I being variablein accordance with a signal at terminal S. Terminal U is the outputterminal. The graphical illustration of the frequency-current relationship of the above modulator is shown in FIG. 2. As suming that f isthe center frequency of the multivibrator, equal changes in currentabout I (-Al to +AI) provide unequal changes (Af,+Af) in frequency.

In FIG. 3, resistance R, has been replaced by a linearization circuitwhose resistance decreases as (I -l-l increases as brought out in theparent application. A decrease in resistance will cause an increase infrequency due to the modification of the associated time constant. DiodeD and transistor T are conductive at the same time and blocked at thesame time. The sudden voltage rise at the collector of T is limited anddefined by the potential difierence between points A and Q whentransistor T is blocked.

In the theoretical expression it can be seen that the term (l -H aperfect linearity cannot be obtained; however, in practice otherphenomena associated with the alteration of the waveform cause linearityto be better than what is theoretically predictable.

Referring to FIG. 4, a symbolic representation of an arrangement of FIG.3 is shown in which it is seen that of the two transistors T and T thefirst has its load resistance replaced by a linearization circuitincluded inside of the rectangle It. This circuit includes a resistanceR in series with a diode D having its anode connected to terminal Awhich may be constituted of a tap on a voltage divider (as shown in FIG.3) inserted at the terminals of a source of supply +E for thetransistors. Since FIG. 4 is shown with all potentials above ground,junctions similar to those in FIG. 3 have been referenced with thereference character of FIG. 3 and further referenced with a prime (i.e.A). A linearization resistance R is inserted between the point of theconnection Q of the diode D and the resistance R supply terminal +E. Iand I of course denote current generators G and G which corresponds totransistors T and T of FIGS. 1 and 3, generator G having it outputvariable, as indicated by the slanted arrow, in response to a modulatingsignal applied at terminal S. The operation of this circuit has justbeen described. FIG. 4 is included in this symbolic form to aid in asimple and clear showing of FIGS. 5 and 6.

In FIG. 5 there is shown a multivibrator according to the presentinvention, in which the circuit shown inside of the rectangle 1 of FIG.4 has been modified and shown as the part included within the rectangle5. As may be seen, between the terminal 4 and the resistance R there hasbeen inserted a reactive circuit 6 including an inductance L and aresistance R connected in parallel with each other. A variablecapacitance C is inserted between the point of connection of thereactive circuit 6 with the resistance R and ground.

The operation of the circuit of FIG. 5 is as follows. Upon the(pplication of a modulating signal at terminal S, the frequency ofoscillation of the multivibrator taken at output terminal U may also bedetermined by Expression 1. The term AV which appears therein representsthe value of the jump in voltage existing between the two triggeringlevels of the multivibrator and may be represented in a firstapproximation as:

V indicates the voltage which is localized at the terminals of the diodein the conduction phase, and V(T) indicates the value of the voltagewhich is localized at the terminals of the diode during the blockedphase.

In the circuit of FIG. 3, the behavior of voltage V(T) as a function oftime is regulated by the behavior of the reverse current involving thediode in the period following its state of conduction, while in the caseof the circuit according to FIG. 5 of the present invention, the desiredbehavior of V (T) as a function of time is realized by appropriatelydimensioning the reactive circuit L R C The regulation of the reactivecircuit is effected by varying capacitance C It would be furtherpossible to maintain G at a fixed value and adjust inductance L In FIG.6 is shown another embodiment of the invention that is similar to thatof FIG. 5, in which however, the linearization circuit 7 is completed bya capacitance C inserted between ground and the end of the reactivecircuit 6 which is opposite to that connected to the capacitance C andfurther by a resistance 0 inserted between the reactive circuit 6 andthe supply terminal +13. The purpose of the inclusion of elements C, andR is to render resistive the impedance presented by the circuit to thebase frequency components which are closed through this path.

Although specific embodiments of the invention have been shown, it isobvious that numerous variants and modifications may be made in theinvention by one skilled in the art without departing from the spiritand scope of the invention and should be included in the appendedclaims:

What is claimed is:

1. A frequency modulator comprising: a multivibrator including first andsecond alternately conducting transistors, each said transistor having abase, an emitter and a collector, a capacitor connected between saidemitters, the frequency of oscillation being dependent on the values ofsaid capacitor, the emitter current of said first transistor and thevoltage change at the collector of said first transistor when it passesfrom a non-conducting to a conducting condition, first and secondcurrent generators respectively connected to said two emitters toprovide linear charging and discharging of said capacitor, said firstcurrent generator including an input for receiving a modulating signal,the output current of said first current generator being dependent onthe amplitude of said modulating signal, an output terminal connected tothe collector of said second transistor, a source of direct currentpotentials coupled to said first and second current generators, and avoltage divider linearizing network connected between said source ofdirect current potentials and the collector of said first transistor,said network including a diode connected in shunt between a firstpotential of said source and said first transistor and poled to beconductive when said first transistor is conductive and non-conductivewhen said first transistor is non-conductive, an inductor connectedbetween a second potential of said source and said diode and said firsttransistor for developing a voltage across said diode when said diodebecomes non-conductive, and a second capacitor shunted to ground betweensaid inductor and said diode, said voltage change at the collector ofsaid first transistor being maintained constant and defined by thepotential change across said diode as it alternates between conductiveand non-conductive conditions.

2. The frequency modulator defined in claim 1, and further comprising athird capacitor connected between the junction of said second potentialof said source with said inductor and ground, the reactance of saidthird capacitor combining with the reactance of said inductor to providethe appearance of a resistive impedance.

References Cited UNITED STATES PATENTS 3,061,799 10/1962 Biard 332-143,127,554 3/1964 Kaneko 332-14 X 3,171,041 2/1965 Haase 307-8853,178,658 4/1965 Henrion.

3,290,617 12/1966 Bellem 332-14 ALFRED L. BRODY, Primary Examiner,

